The field of the invention is valves with a failsafe mode of closure for oilfield use, primarily in sub-sea applications and more particularly, in the preferred embodiment, which compensate for a rise in internal pressure around the gate when opening and allow internal line pressure to assist in valve closure.
Valves used in sub-sea drilling applications have had actuators with fail-safe closure provisions. Generally, the force required to return the actuator piston and the valve to a fail safe position, which, in most cases were the fail closed position is from the spring force and the actuator stem force. The spring force is normally relatively low in comparison to the total force required for fail-safe operation. The actuator stem force is a primary fail-safe force presented a net area of the stem cross-sectional area that was exposed internally to the valve body. Generally a spring or springs were used to return an actuating piston and the valve gate to a fail-safe position, which, in most cases was the closed position. In some designs, the valve actuator stem presented a net area exposed to internal valve pressure, which, in the absence of hydraulic pressure on the actuating piston provided a net force to move the gate to its fail-safe position. These large unbalanced forces were needed to overcome gate drag due to internal pressures in the valve body forcing the gate laterally. The return spring would also act on the actuating piston to urge the gate to the fail-safe position.
In drilling applications a condition could exist where the valve body is full of an incompressible fluid like drilling mud. When trying to stroke the gate from a closed to an open position, the stem connecting the gate and the actuating piston would enter the valve body. If the valve body was full of an incompressible fluid, the internal pressure could rise to the point that the maximum working pressure of the valve body could be exceeded. Additionally, further movement of the gate could be stalled as the pressure buildup around the gate could rise to the level where the hydraulic system acting on the actuating piston could not overcome the built up internal pressure from the surrounding incompressible fluid. To compensate for this effect, a balancing stem was attached to the lower end of the gate, to minimize or eliminate this pressure buildup that would otherwise occur as the valve is actuated to open. However, the addition of the balancing stem attached to the gate solved one problem but created another. Since the gate was essentially in pressure balance from internal valve pressure a net unbalanced force was no longer available to overcome gate drag when a fail-safe operation was required. Normally, the return springs could only put out a few thousand pounds of force to assist in the fail-safe movement, but to overcome gate drag forces well in excess of 25,000 pounds would be needed. The solution to the problem was to design an auxiliary pressurized accumulator, which could take the place of the force formerly provided by internal pressure acting on a net area of the gate assembly to drive it to the fail-safe position. The accumulators were large and heavy and their required size and weight increased with the sub-sea depth of the application. They also presented safety concerns in that their pressure had to be released to equalize with the sub-sea pressure before being brought to the surface. They also presented safety concerns in that their pressure had to be vented prior to actuator disassembly to avoid injury to maintenance personnel.
Various designs of sub-sea drilling gate valves have been attempted, some with the pressure balanced feature, as shown in U.S. Pat. Nos. 4,809,733; 4,311,297; 4,230,299; 4,489,918; U.S. Pat. No. Re 29,322; U.S. Pat. Nos. 4,281,819; 6,125,874; and U.S. Pat. No. Re 30,115. Of these, the latter two are of most interest as they provide a way to use the surrounding seabed pressure to urge a balancing piston against the gate to make the valve fail-safe. However, even these two latter references do not provide the ability to compensate for a buildup in internal pressure around the gate during opening while at the same time having a provision to allow a net internal pressure to act on an unbalanced gate to achieve a fail-safe position. In the present invention large accumulators are eliminated or minimized. A compensating piston, which is biased toward the gate but not connected to it, is used in the preferred embodiment. A self contained, charged, pressure chamber acts on the compensating piston. An easy retrofit of existing valves is also possible. These and other advantages of the present invention as well as additional features will be more readily appreciated by those skilled in the art from reading the description of the preferred embodiment, which appears below.
A fail-safe gate valve for sub-sea use features a floating, pressure biased compensating piston whose movement prevents internal pressure buildup from opening movement of the gate. A pre-charged fluid chamber provides the bias on the balancing piston. Using unequal piston diameters reduces the charge pressure. The balancing piston is not connected to the gate so that internal pressures can be employed to act on a net area, which biases the gate toward its fail-safe position.